1. Field of the Invention
The invention relates to a radar-based method of measuring the level of a material in a vessel in which an antenna arranged above the highest level anticipated radiates electromagnetic waves to the surface of the material contained in the vessel and receives reflected echo waves and in which the transit time corresponding to the group delay of the wanted echo waves reflected by the material surface is measured, from which the level is computed.
2. Description of the Prior Art
In prior art methods of this kind, level measuring is based on the product of the measured transit time and the velocity of propagation of the electromagnetic waves equalling twice the distance between the antenna and the material surface, since this distance is travelled twice by the electromagnetic waves. The level is thus given by the difference between the known installation height of the antenna and the distance as determined by measuring.
The prerequisite for this method of measuring is that the velocity of propagation of the electromagnetic waves is known, this being assumed in most cases to be the speed of light in air. This assumption is admissible when the transverse dimensions of the space in which the electromagnetic waves propagate are large as compared to the wavelength of the electromagnetic waves. Since the resolution of level measuring depends on the wavelength of the electromagnetic waves this is usually done with microwaves so that the above prerequisite is satisfied as a rule where large vessels are concerned.
When, however, measuring is done in a tube inserted in the vessel and the tube has a diameter which is small as compared to the wavelength, for example in a so-called surge tube, or when the vessel itself has a correspondingly small diameter, the above assumption is no longer admissible. A tube or vessel of small diameter in which electromagnetic waves propagate acts like a waveguide. The group velocity of electromagnetic waves in a waveguide depends on the geometry thereof and is smaller by a factor, termed the reduction factor, than the speed of light. The group velocity is the velocity at which the energy is transferred in the waveguide. When measuring the transit time is done, as usual, on the basis of the amplitude of the reflected echo waves the measured transit time is the group delay. The reduction factor is typically of the order of 0.7 to 1.0, and thus ignoring the reduction factor, i.e. the assumption that it always equals 1.0 may thus result in serious measuring errors. This is why it is particularly in the case of high-accuracy measuring that precisely knowing the reduction factor is important.
In conventional methods of level measuring using microwaves in which the reduction factor is to be taken into account, a calibration is done before commencing measuring, this involving measuring the transit time with the vessel empty or with a precisely known level. However, this calibration is not possible when measuring needs to be commenced with the vessel partly filled, the level of which is not known precisely. Furthermore, once-only calibration may not be sufficient to establish whether or not the reduction factor changes in the course of time or differing reduction factors exist in various portions of the vessel.